Integrated circuit (IC) "chips" comprising very large numbers of electronic components have become ubiquitous in modern society. Electronic devices and components of all sorts, from central processing units used in all levels of computing, to highly specialized controllers used to control various types of equipment and machinery, are now routinely available as integrated circuit chips. Since the introduction of the first IC chips, there has been a remarkable increase in the number of devices contained on a single chip, as well as a corresponding dramatic reduction in the size of the individual electronic components formed on the chip. Device geometries with line widths of the order of one micron have become common so that individual IC chips now routinely contain in excess of a million electronic components. Even higher device densities are projected.
The increase in device complexity and the decrease in device size has, for many types of IC chips, sharply increased the complexity of forming interconnections between the chips and external devices.
Many devices, such as computers, utilize a large number of separate IC chips. For example a computer may have one or more central processing unit (CPU) chips, various memory chips, controller chips, input/output (I/O) device chips, etc. Traditionally, each chip is mounted in a separate package which is then connected to a printed circuit board, for example, a computer "motherboard," which supplies power to the chip and provides signal routing among the chips on the board and to various I/O devices. However, where an electronic device utilizes a substantial number of chips, packaging each chip separately greatly increases the total area of printed circuit board needed to interconnect all the chips. In addition, as device speed has increased, the distance between individual components has become an increasingly important factor, so that it is important, in many applications, to minimize the signal path between IC chips used in the system.
In order to overcome the aforementioned problems, many device makers have begun using "multichip modules," (sometimes abbreviated "MCM"), i.e., packages housing a plurality of individual IC chips. Typical multichip modules incorporate not only means for interconnecting the IC chips with external devices, but also means for interconnecting the IC chips within the module. A general introduction to multichip modules, including a discussion of the history of the development thereof, is described in the text entitled: Multichip Module Technologies and Alternatives, The Basics, D. A. Doane, et al., eds., Van Nostrand Reinhold (1993). Multichip modules significantly reduce the overall space needed to house the IC chips and, by shortening the distance between chips within the module, facilitate high speed device operation.
The first multichip modules were two-dimensional, i.e., all of the IC chips housed in the package were mounted on a planar substrate. Subsequently, three-dimensional multichip modules were developed, thereby permitting an even further increase in the density of IC chips that could be housed in a single package. However, placing a large number of high density chips in close proximity greatly complicates the task of supplying power to and routing signals to and from the chips. In view of the complicating factors associated with three-dimensional arrays, two-dimensional multichip arrays remain the most common form of multichip modules in use today.
Two major substrate technologies have been developed for handling the power supply and signal routing in multichip modules. Initial multichip module designs utilized co-fired ceramic substrate technology. The trend in recent years has been a shift to "thin film" substrate technology. In some cases, the two technologies have been combined to produce hybrid chip modules using both thin film and ceramic layers. In all multichip module designs, a plurality of IC chips are connected to a multilayered substrate which contains the signal and power lines needed to supply power and to interconnect the chips to each other and to external devices. In order to make the required number of interconnections, such substrates are multilayered, sometimes containing dozens of individual layers. For example, even early ceramic substrate technology utilized as many as thirty-five separate layers in the multichip substrate. However, problems arise in placing signal lines in close proximity to each other and to power supply lines. The dielectric constant of the substrate material plays an important role in solving (or creating) these problems. One of the reasons ceramic technology has lost favor is due to the high dielectric constant associated with the ceramic materials typically used as a substrate material. Thin film substrates made of materials such as polyimide or other polymers have become more common due, in part, to the much more favorable dielectric properties of these materials. In addition, the processing techniques used with polyimides allow the creation of much finer structures, thereby more easily accommodating higher device densities.
It is well known that it is important and desirable to provide bypass capacitance in close proximity to the integrated circuit chips in a multiple chip module, and the need for such capacitance increases as the switching speed of the devices becomes higher. In some designs, bypass capacitance is incorporated into the multilayered MCM substrate by forming capacitor plates within the MCM substrate. This technique adds complexity to the multilayered substrate, reducing the manufacturing yield. Another approach is to mount discrete capacitors as separate components on the surface of the MCM substrate. However, this arrangement uses up valuable "real estate" on the surface of the MCM and suffers from the fact that such capacitors are not as close to the IC chips as needed.
The approach of the present invention is to overcome these problems by using what will be referred to herein as an interposer substrate which is positioned between the main multilayered, MCM substrate and an integrated circuit chip. Such interposer substrates, which may incorporate embedded capacitors, are mounted on the MCM substrate, with the IC chips being mounted on the interposer substrates. This arrangement allows the bypass capacitance to be positioned extremely close to the IC chip, and increases the modularity of the overall MCM, thereby improving overall system yields and reducing manufacturing costs. The interposer substrates can be separately manufactured and tested before incorporation into a multichip module. This is especially important since the capacitor structure is one of the most likely components to be defective, due to the relatively close spacing of the plates and the possibility of pin-hole defects or other causes of electrical shorting or leakage in the thin dielectric layer between the plates. If a defective capacitor is incorporated into a MCM substrate and not discovered until fabrication of the substrate is complete, the resulting loss may be quite significant.
Interposer substrates have also been used in the past to accommodate differences in the thermal expansion coefficients between the MCM substrate and the IC chip.
High speed device operation often also requires the use of terminal resistors positioned very close to the IC chips to match impedance between the chips and the signal lines carrying data to and from the chips. As is well known in the art, close impedance matching enhances power transfer and avoids problems associated with reflections of the signals. It is likewise important that the impedance of the signal lines be controlled.
Another problem with traditional approaches to packaging IC chips in MCM's is the method used for delivering power to the chips. One aspect of this problem results from routing power lines through the same substrate utilized to carry signals to and from the chip. Equally important is the fact that the thinness of the substrates used in traditional multichip modules results in power feeds to the IC chips that have relatively high impedance. This high impedance results in undesired noise, power loss and excess thermal energy production. These same problems are applicable to routing power and signal lines though an interposer substrate.
Accordingly, an object of the present invention is to provide an interposer substrate for use in coupling integrated circuit chips to a multichip module substrate which incorporates bypass capacitance.
Another object of the present invention is to provide an interposer substrate, as above, which incorporates terminal resistors.
A further object of the present invention is to provide an interposer substrate which provides controlled impedance signal paths for the lines carrying signals from a multichip module substrate to an integrated circuit chip.
Yet another object of the present invention is to provide an interposer substrate which substantially isolates the routing of signal lines from the routing of power lines from a multichip module substrate to an integrated circuit chip.
Still another object of the present invention is to provide a method of manufacturing an interposer substrate having the foregoing characteristics at a reasonable cost.
A further object of the invention is to reduce voltage drops in the power lines by providing a separate power plate in which the interposer substrates can be embodied.